As stated elsewhere on our site, there’s no ‘magic’ to creating a super-efficient, comfortable home. Technique trumps technology, and if you had to choose between a carefully built shell or an exotic gadget, the shell wins every time. Nearly everything we’ve used in the construction of our house is readily available on the market today, and the few things we’ve had to import were brought here for their superior performance, not their alien technologies. That said, our tours invariably elicit the most questions about the heat recovery ventilator, or HRV. Windows come in second place, followed by our toilets. Seriously.
The HRV is the lungs of the house, allowing fresh air to be brought into every room of the house while stale air is removed. Without it, the air-tight shell would create a bubble of increasingly humid, smelly and CO2-rich air. This isn’t only dangerous for people, but for the entire structure. Humidity, above all other things, must be controlled for both human comfort and to prevent condensation from forming in the walls. Because of this, you must exchange interior air with exterior air whenever you are cooling or heating the house.
How does it work?
HRVs work via a heat exchanger – as warm, humid, stale air is moved out of the home, it is run through an exchanger against the cold, dry, fresh air outside. The result is the exiting air warming the incoming air, so the fresh air being introduced to the home is just a few degrees cooler than the house’s ambient air temperature. This radically reduces the amount of heat exiting the shell, which means that you get all of the benefit of that pure winter air, but none of the energy implications of just piping it in.
Because the flow of air must be balanced – there must be exact same amount of air coming in as going out – there are certain considerations in how the system is laid out. The most important is understanding what rooms you want to move air out of (bathrooms, kitchen, utility rooms) and what rooms you want air moving into (living rooms, bedrooms, offices). Ultimately fresh air moves into every room, but it’s always flowing from the rooms you live in to the rooms you use only intermittently.
The second understanding is that because the house’s air is always balanced, you cannot use exhaust-only vents. No vents in the bathroom to suck air out of the room and dump it outside, no dryer vent in the washroom, no range vent in the kitchen. These all present technical challenges, but with the HRV doing most of the work and with condensing technologies, they can all be addressed in a way that does not impact air quality or daily life.
There’s a fairly wide range of performance in HRVs available to builders and home owners. Typical performance is between 70 and 80% recovery, which means that about one third to one fifth of the heat from the exhaust air is never recaptured. That increases your heating bill, impacts comfort, and reduces the overall efficiency of the home.
We installed a Passivhaus certified HRV from Paul (a Swiss company, now owned by Zehnder), which has a recovery rate of 95%. During construction we very simply tested this by measuring the temperature of the air being exhausted (we kept the house around 65F this winter) and the fresh air being supplied to the living space. There was about a two degree difference, which is scarcely noticeable.
The actual heat exchanger and air manifolds are HUGE, which meant that we had to sacrifice a fair amount of space in our utility room to accommodate them. The HRV is larger than your typical unit, in part due to the amount of insulation it has, and in part due to the massive heat exchanging core that sits at its heart. The manifold is not just a manifold, but also a silencer. It looks like an empty metal box, but has foam covered channels that eliminate any sounds that could be transmitted from the HRV.
Living with an HRV
The results are amazing. Even when the HRV is running flat out, exchanging nearly 200 CFM, the utility room is quiet enough to carry on a normal conversation. It’s inaudible in any other room of the house. At normal speeds, the intake and exhaust ports in each of the rooms are utterly silent, only making enough sound to hear if you’re pressing your ear up to the end of the outlet. There’s also no perception of air movement (something I truly hate about forced-air heating systems), leaving every room silent and still while the system mysteriously does its work.
A secondary benefit of the HRV is that it helps to eliminate temperature differences from room to room. With super-insulated shells this is already a lesser issue, but the constant mixing and moving of air from room to room seems to negate this phenomenon entirely. Even on very cold days (Minneapolis got to about -5F this winter), we’ve found that there was only a five degree difference from the room with the space heater and a room on the other side of the house, on a different floor. Once we have the in-floor heat running and heating every square foot of the home, that difference should be close to zero.
Nothing is perfect, and as mentioned above, there are some drawbacks to ultra-high efficiency HRVs in a Passive House.
Our HRV is big, and takes up a lot of room in what is a rather small house. The ductwork is simple to install and hide in walls, but any time you have to run fifteen 3″ tubes from the basement to every room in the house, it’s going to complicate things. Fortunately, Tim designed a mechanicals wall into the plans (it accommodates literally everything that runs between floors), which greatly eased the pain.
Paul and Zehnder HRVs are imported from Europe, and are very expensive. Like most things, you get what you pay for, but the cost of materials alone were probably double what we would have paid for a product from a domestic vendor. Having products imported from overseas also increases technical support windows; forget calling your local HVAC contractor to get replacment parts or service the next day. I’ve spoken to Zehnder about this, and they have plans in place to provide local support in the near future.
Finally, and related to the above, the approach to separating ventilation from heating and cooling is relatively new to the US. Getting an installer to work with dedicated HRV ductwork can be a challenge. This is NOT because it’s hard to do (my friend Jason and I installed most of the system in a weekend), but because contractors dislike working with new techniques and technologies. The building industry is extremely conservative by its nature, and changes slowly.
All of that said, would we do things differently? Probably not. While there was a fair amount of pain getting the system up and running correctly, now that it’s fully functional we’re finding that our ventilation system is everything we could have hoped for. Now we’re looking forward to years of quiet, reliable service to come.